Human monoclonal antibody AVP-21D9 to protective antigen reduces dissemination of the Bacillus anthracis Ames strain from the lungs in a rabbit model.

Dutch-belted and New Zealand White rabbits were passively immunized with AVP-21D9, a human monoclonal antibody to protective antigen (PA), at the time of Bacillus anthracis spore challenge using either nasal instillation or aerosol challenge techniques. AVP-21D9 (10 mg/kg) completely protected both rabbit strains against lethal infection with Bacillus anthracis Ames spores, regardless of the inoculation method. Further, all but one of the passively immunized animals (23/24) were completely resistant to rechallenge with spores by either respiratory challenge method at 5 weeks after primary challenge. Analysis of the sera at 5 weeks after primary challenge showed that residual human anti-PA levels decreased by 85 to 95%, but low titers of rabbit-specific anti-PA titers were also measured. Both sources of anti-PA could have contributed to protection from rechallenge. In a subsequent study, bacteriological and histopathology analyses revealed that B. anthracis disseminated to the bloodstream in some naïve animals as early as 24 h postchallenge and increased in frequency with time. AVP-21D9 significantly reduced the dissemination of the bacteria to the bloodstream and to various organs following infection. Examination of tissue sections from infected control animals, stained with hematoxylin-eosin and the Gram stain, showed edema and/or hemorrhage in the lungs and the presence of bacteria in mediastinal lymph nodes, with necrosis and inflammation. Tissue sections from infected rabbits dosed with AVP-21D9 appeared comparable to corresponding tissues from uninfected animals despite lethal challenge with B. anthracis Ames spores. Concomitant treatment with AVP-21D9 at the time of challenge conferred complete protection in the rabbit inhalation anthrax model. Early treatment increased the efficacy progressively and in a dose-dependent manner. Thus, AVP-21D9 could offer an adjunct or alternative clinical treatment regimen against inhalation anthrax.

Murine macrophage transcriptional and functional responses to Bacillus anthracis edema toxin.

Edema toxin (EdTx), which is a combination of edema factor and a binding moiety (protective antigen), is produced by Bacillus anthracis, the etiological agent of anthrax. EdTx is an adenylyl cyclase enzyme that converts adenosine triphosphate to adenosine-3',5'-monophosphate, resulting in interstitial edema seen in anthrax patients. We used GeneChip analysis to examine global transcriptional profiles of EdTx-treated RAW 264.7 murine macrophage-like cells and identified 71 and 259 genes whose expression was significantly altered by the toxin at 3 and 6h, respectively. Alteration in the expression levels of selected genes was confirmed by real time-reverse transcriptase polymerase chain reaction. The genes with up-regulated expression in macrophages in response to EdTx-treatment were known to be involved in inflammatory responses, regulation of apoptosis, adhesion, immune cell activation, and transcription regulation. Additionally, GeneChip analysis results implied that EdTx-induced activation of activator protein-1 (AP-1) and CAAAT/enhancer-binding protein-beta (C/EBP-beta). Gel shift assays were therefore performed, and an increase in the activities of both of these transcription factors was observed within 30 min. EdTx also inhibited tumor necrosis factor alpha production and crippled the phagocytic ability of the macrophages. This is the first report detailing the host cell global transcriptional responses to EdTx.

Human monoclonal anti-protective antigen antibody completely protects rabbits and is synergistic with ciprofloxacin in protecting mice and guinea pigs against inhalation anthrax.

Prevention of inhalation anthrax requires early and extended antibiotic therapy, and therefore, alternative treatment strategies are needed. We investigated whether a human monoclonal antibody (AVP-21D9) to protective antigen (PA) would protect mice, guinea pigs, and rabbits against anthrax. Control animals challenged with Bacillus anthracis Ames spores by the intranasal route died within 3 to 7 days. AVP-21D9 alone provided minimal protection against anthrax in the murine model, but its efficacy was notably better in guinea pigs. When Swiss-Webster mice, challenged with five 50% lethal doses (LD50s) of anthrax spores, were given a single 16.7-mg/kg of body weight AVP-21D9 antibody dose combined with ciprofloxacin (30 mg/kg/day for 6 days) 24 h after challenge, 100% of the mice were protected for more than 30 days, while ciprofloxacin or AVP-21D9 alone showed minimal protection. Similarly, when AVP-21D9 antibody (10 to 50 mg/kg) was combined with a low, nonprotective dose of ciprofloxacin (3.7 mg/kg/day) and administered to guinea pigs for 6 days, synergistic protection against anthrax was observed. In contrast, a single dose of AVP-21D9 antibody (1, 5, 10, or 20 mg/kg) but not 0.2 mg/kg alone completely protected rabbits against challenge with 100 LD50s of B. anthracis Ames spores, and 100% of the rabbits survived rechallenge. Further, administration of AVP-21D9 (10 mg/kg) to rabbits at 0, 6, and 12 h after challenge with anthrax spores resulted in 100% survival; however, delay of antibody treatment by 24 and 48 h reduced survival to 80% and 60%, respectively. Serological analysis of sera from various surviving animals 30 days postprimary infection showed development of a species-specific PA enzyme-linked immunosorbent assay antibody titer that correlated with protection against reinfection. Taken together, the effectiveness of human anti-PA antibody alone or in combination with low ciprofloxacin levels may provide the basis for an improved strategy for prophylaxis or treatment following inhalation anthrax infection.